Improve CMAKE_OSX_DEPLOYMENT_TARGET and CMAKE_OSX_SYSROOT handling

[Xottab-DUTY]

Currently, we are setting CMAKE_OSX_DEPLOYMENT_TARGET and CMAKE_OSX_SYSROOT variables after the project call which is not recommended.
They should be set externally by the developer invoking CMake.

xray-16/cmake/XRay.Compiler.GNULike.cmake

Line 9 in 9352fe4

set(CMAKE_OSX_DEPLOYMENT_TARGET ${CMAKE_SYSTEM_VERSION})

CMAKE_OSX_SYSROOT is even more problematic: it is hardcoded with the assumption that Xcode is installed.

xray-16/Externals/LuaJIT-proj/CMakeLists.txt

Line 18 in 9352fe4

set(CMAKE_OSX_SYSROOT "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk")

But it may be missing and a developer can have command line tools installed instead and the path would be Library/Developer/CommandLineTools/SDKs/MacOSX.sdk

https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_DEPLOYMENT_TARGET.html
https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_SYSROOT.html